Method of treating iron powder



fi 1969 K. a. THAFVELIN ET 3,462,26

METHOD OF TREATING IRON POWDER Filed March 1. 1966 IIII I I I I I I I I01234 012 3 4 5 6 7 8luH JNVENTORS Karl Gunnar Thaf L/H Yn ve Wa LberGeorg emnch Avtlgge z g r ia United States Patent O 3,462,260 METHOD OFTREATING IRON POWDER Karl Gunnar Thafvelin, Yngve Wahlberg, and GeorgHeinrich Artur Gerhard Bockstiegel, Hoganas, Sweden, assignors toHoganas-Billesholms Aktiebolag, Hoganas, Sweden, a company of SwedenFiled Mar. 1, 1966, Ser. No. 530,896 Claims priority, applicationSweden, Mar. 9, 1965, 3,07 9/ 65 Int. Cl. 1322f N US. Cl. 75.5 2 ClaimsABSTRACT OF THE DISCLOSURE The green strength of a reduced iron powderthat has been cold-worked to improve its compressibility is increased byfirst oxidizing the powder to a H -loss of 1- 10% and then reducing theoxidized powder.

Iron powder having been manufactured by a reduction process at atemperature below the melting point with a subsequent pulverization andsieving is characterized by an irregular particle shape. When such apowder is pressed and sintered to form shaped bodies said irregularparticle shape produces a high green strength, that is the strength ofthe pressed body before having been sintered. If the powder is pressedat a medium pressure, however, it will have a comparatively lowcompressibility, measured as g./cm. of the pressed, unsintered body,also called green density. This is, among other things, due to theporosity of the powder.

Efforts have been made of improving the compressibility of such powdersby a cold-working process, for instance a rolling process, with asubsequent pulverization to the desired particle size, and annealing toremove the hardening produced by the cold-working. This cold-workingprocess has improved the compressibility but has also reduced the greenstrength considerably.

Therefore, this invention is concerned with the problem of increasingthe compressibility of a porous powder and finding a method forincreasing its green strength without significantly reducing theobtained compressibility.

Said problem is solved by subjecting the powder to a limited oxidationafter the cold-Working process, for instance the rolling process, saidlimited oxidation being followed by a reduction in hydrogen.

The new method for treating iron powder for increasing thecompressibility while maintaining a satisfactory green strength ismainly characterized in subjecting the powder to one or morecold-working processes, for instance rolling processes, if desiredseparated by annealing processes, pulverizing the powder to the desiredparticle size, subjecting the powder to an oxidation process until thepowder has absorbed a quantity of oxygen corresponding to a socalled H-loss of 1-10%, preferably approximately 5%, and reducing the powder inhydrogen.

The H -loss is a term used within the iron powder metallurgy to indicatethe degree of reduction or the quantity of residual oxygen, and isdetermined as the loss in weight, in percent, occurring when a powder isannealed in hydrogen at 1050 C. for 60 minutes.

The information will now be described more in detail with reference toan example in which a powder is made 3,462,260 Patented Aug. 19, 1969from sponge iron manufactured according to the Hoeganaes method.

The sponge iron is crushed to a particle size below 3 mm. This powder issubjected to a first rolling process. The product from the rollingprocess consists of substantially coherent strip-shaped portions. Thisproduct is crushed, preferably in a disintegrator. The rolling andcrushing processes are repeated four times. Samples are taken after eachcrushing operation.

After the last crushing operation the material was divided intoportions, and said portions were subjected to oxidation processesresulting in various values of the H loss, namely 0.55%, 5.24%, and7.95%. The oxidated portions were reduced in dry hydrogen at 960 C. for60 minutes. The reduced portions were cautiously pulverized to aparticle size of less than 100 mesh, that is less than 0.15 mm. Thecompressibility of the powders thus produced was ascertained by pressingthe powders at a pressure of 4.2 ton/cmF.

The result is illustrated on the accompanying drawing.

In FIG. 1 the ordinate represents the green density, measured as g./cm.The abscissa is divided into two portions. The left-hand portionrepresents the number of cold-working operations. The right-hand portionrepresents the degree of oxidation, measured as the so-called H -loss.

FIG. 1 clearly shows how the green density increases with the number ofcold-working operations, and how it first continues to increase after amild oxidation-reduction operation, but decreases as a result of anincreased oxidation. An oxidation to the H -loss of 8% with subsequentreduction produces a compressibility which is equal to that producedafter one cold-working operation. Therefore, we prefer to keep the H-loss value below 8%.

In FIG. 2 the ordinate represents the green strength, measured as poundsper square inch. The abscissa is divided into two portions similiarly toFIG. 1. The three curves in FIG. 2 represent test bodies having beenpressed to various densities, viz. 6.0, 6.5 and 7.0 g./cm. respectively.FIG. 2 illustrates the serious decrease in green strength as a result ofthe rolling operations, and how the green strength can again beincreased by a subsequent oxidation-reduction operation.

We claim:

1. Method of increasing the compressibility of a porous cold-worked ironpowder and simultaneously increasing its green strength withoutsignificantly reducing its compressibility, which comprises oxidizingthe powder to an Hg'lOSS value within the range from 1-10% and thenreducing the oxidized powder.

2. Method as defined in claim 1 in which the H -loss value of theoxidized iron powder is about 5%.

References Cited UNITED STATES PATENTS 2,558,750 7/1951 Harrison 0.552,853,767 9/ 1958 Burkhammer 75211 2,902,357 9/ 1959 Crooks et al. 750.5

L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLA-RD, AssistantExaminer US. Cl. X.R. 75--2l1; 148--l26

